1. Field of the Invention
This invention relates generally to the fabrication of semiconductor devices and acousto-optical structures and, more particularly, to a low temperature process for depositing an epitaxial layer of a chosen material on a selected substrate.
2. Description of the Prior Art
In the fabrication of certain semiconductor devices and circuits, selected materials are sometimes deposited on an underlying substrate in the form of an epitaxial layer, which is a layer whose crystal orientation is induced by the substrate. The epitaxial layer may be either single crystal or polycrystalline. The substrate may be of the same or different material and structure as the deposited layer, and the substrate provides, through its lattice structure, preferred positions for the deposition of the epitaxial material. The result is that the epitaxial material forms an extension of the crystal structure of the substrate, which produces desirable electrical and optical properties in a device formed of such materials.
Epitaxial materials are used in a variety of semiconductor devices, such as: silicon-on-sapphire. (SOS) devices in which epitaxial silicon is formed on a sapphire substrate; infrared detectors, in which epitaxial mercury cadmium telluride is formed on a cadmium telluride substrate; avalanche diode detectors in which epitaxial germanium is formed on germanium or gallium arsenide substrates. In addition, epitaxial zinc oxide on a sapphire substrate is useful as an optical waveguide and in acousto-optical applications; and epitaxial lead sulfide on a silicon substrate provides the active element in infrared detectors and infrared charge-coupled devices.
In the past, the growth of epitaxial layers of various materials has been attempted by using such processes as sputtering, evaporation, high temperature liquid or vapor phase epitaxial growth, chemical vapor deposition, or a water solution chemical reaction. More specifically, oxide layers and sulfide layers have been formed using known non-reactive or reactive sputtering techniques. In a known non-reactive sputtering process, a disk of the selected material to be deposited, such as zinc oxide (ZnO) or zinc sulfide (ZnS), is bombarded in a reaction chamber with inert ions, such as argon ions, which cause the ZnO or ZnS to vaporize from the target (disk); and the vaporized ZnO or ZnS subsequently deposits on the selected substrate . Using a known reactive sputtering process, such as described by Maniv and Zangvil in the publication entitled "Controlled texture of reactively rf-sputtered ZnO thin films," in the Journal of Applied Physics, Vol. 49, No. 5, May 1978, pages 2787 to 2792, a disk of the selected metal, such as zinc, is bombarded in a reaction chamber with oxygen ions, which causes vaporization of the zinc from the target, and the vaporized zinc and oxygen ions then react to produce the desired oxide, which deposits on the selected substrate. However, such sputtering techniques produce polycrystalline ZnO films with crystalline orientations independent of the orientation and nature of the substrate, that is, growth is not epitaxial. In addition, ZnO films formed by sputtering often have non-smooth surfaces, which causes undesirable optical propagation losses. Further, it has been observed that polarity inversion of adjacent crystallites occurs in sputtered ZnO films, which causes reduced piezoelectric efficiency and makes such films unsuitable for acousto-optical applications.
Moreover, in both the reactive and the non-reactive sputtering processes, the bombarding ions are formed by subjecting the chosen bombarding material, such as oxygen or argon, respectively, to a radio frequency (rf) or direct current (dc) discharge. However, as a result of the exposure of the chosen bombarding material to discharge, numerous extraneous ionized and neutral particles and high energy radiation with wavelengths as low as 500 angstroms or lower are produced. These extraneous particles then bombard the surface of the substrate on which the oxide is being formed and cause damage thereto by altering the quantity and distribution of charge therein. In addition, the bombardment of the substrate surface by these particles causes the formation of additional charged particles and radiation, which may also damage the substrate. This alteration in the charge of the substrate undesirably alters the electrical performance of the substrate and any structures formed therein. The damage produced by charge bombardment and radiation bombardment is particularly noticeable when the substrate comprises an electrically sensitive device, such as a charge coupled device or a device formed of certain compound semiconductor materials, such as mercury cadmium telluride or indium antimonide.
Using a known evaporation process to form an oxide or a sulfide layer, such as zinc oxide or zinc sulfide, a source comprising the selected oxide or sulfide is placed in a reaction chamber and is raised to an elevated temperature sufficient to cause evaporation of the oxide or sulfide, which subsequently deposits on the selected substrate. Alternatively, a known reactive evaporation process may be used in which a metal source is evaporated and the evaporant is reacted with oxygen at the substrate surface to form an oxide layer. However, these evaporation processes produce ZnO films which are not epitaxial, have non-smooth surfaces, and polarity inversions, which have the disadvantages previously discussed with regard to sputtering techniques.
Using a known thermal chemical vapor deposition (CVD) process for forming an oxide layer, a metalorganic compound, such as dimethyl zinc, is thermally decomposed at the heated substrate surface in the presence of an oxygen source to produce the desired zinc oxide. However, such thermal CVD processes typically employ temperatures in excess of 500.degree. C., which are not compatible with certain temperature-sensitive compound semiconductor materials, such as mercury cadmium telluride, which undergo decomposition at elevated temperatures.
Yet another known process for the epitaxial growth of layers of compound semiconductor materials is a liquid phase epitaxial growth process, such as described in U.S. Pat. No. 4,238,252 to Kamath and Holmes and assigned to the present assignee. In such a process, an epitaxial layer of a chosen material, such as indium phosphide or another material containing elements of Groups III and V of the periodic table, is grown by first providing a crystal growth solution of material containing the chosen elements, such as a solution of indium saturated with phosphorus. The melt is maintained in a reaction chamber at a temperature above the solution liquidus temperature (such as 750.degree. C. for the indium and phosphorus solution). Then, the chosen substrate, such as an indium phosphide substrate, is contacted with the growth solution while the solution is cooled below its liquidus temperature. This cooling initiates the single crystal growth of the epitaxial layer of indium phosphide. However, such liquid phase epitaxial growth processes often produce epitaxial layers having high surface defect densities and voids, which result in degraded device performance. For example, point defects can lead to unwanted charge emission under high electric field conditions; and large pinholes or voids can result in poor dielectric step coverage, which causes reduced breakdown voltages or high leakage current. In addition, material grown by a liquid phase epitaxial process tends to have a degree of surface roughness which causes major optical propagation losses in acousto-optical structures and which requires that the surface of such structures be polished. Moreover, at the high temperature required in a liquid phase epitaxial growth process, an unwanted reaction can occur between the epitaxial layer, such as zinc oxide, and the substrate, such as sapphire. Further, this high temperature causes unwanted autodoping effects, in which dopants from the substrate become incorporated in the epitaxial material and alter the electrical properties thereof.
Using a known vapor phase epitaxial growth process, such as described in U.S. Pat. No. 3,657,004 to Merkel et al, an epitaxial layer is grown on the surface of a chosen substrate by exposing the substrate at high temperature to chosen vapor phase reactants. This vapor phase process uses a chemical transport reaction in which a solid or liquid substance "A" reacts with a chosen gas to form only gaseous reaction products. These products are then transported to the substrate surface where the reverse reaction occurs and the substance "A" deposits on the substrate. For example, in the process of Merkel et al, water and boron trioxide provide the transport medium for gallium arsenide at a temperature of 700.degree. to 775.degree. C. The vapor phase epitaxial growth of zinc oxide has been accomplished as described, for example, by C. K. Lau et al, in the publication entitled "Growth of Epitaxial ZnO Thin Films by Organometallic Chemical Vapor Deposition," the Journal of the Electrochemical Society, Vol. 127, No. 8, 1980, pages 1843 to 1847, using diethylzinc with H.sub.2 O/H.sub.2, N.sub.2 O/N.sub.2, and CO.sub.2 /H.sub.2 oxidizing gas systems at temperatures of 400.degree. C. and 730.degree. C. However, the high temperature required for these vapor phase epitaxial growth processes causes unwanted reaction between the epitaxial layer and the substrate and unwanted autodoping, as previously discussed with regard to liquid phase epitaxial processes. Moreover, in such a vapor phase growth process, competing vapor phase reactions may occur and form zinc oxide, for example, without substrate nucleation, which becomes incorporated in the epitaxial film and reduces the crystalline quality thereof.
Finally, a water solution chemical reaction has been used to deposit an epitaxial layer of lead sulfide on a germanium substrate, as described by Davis and Norr in the publication entitled "Ge-Epitaxial-PbS Heterojunctions," in the Journal of Applied Physics, Vol. 37, No. 4, Mar. 15, 1966, pages 1670 to 1674. By the process of Davis et al, a water solution of sodium hydroxide and lead nitrate is prepared; the germanium substrate is immersed in the solution; thiourea is added to the solution; and the solution is allowed to sit at room temperature for 30 minutes, at the end of which the substrate has been coated with epitaxial lead sulfide. However, such solution processes tend to form layers which are highly polycrystalline and have poor surface morphology, both of which properties tend to degrade device performance.
In all of the above-described processes, the selection of the substrate material on which to deposit an epitaxial material depends on two important factors: (a) the extent of lattice mismatch between the substrate and the material to be deposited; and (b) the difference in the thermal coefficients of expansion of the substrate and of the material to be deposited. It is desirable to have minimized lattice mismatch so that the deposited material will be able to replicate the crystal structure of the substrate. It is also desirable to have closely matched thermal coefficients of expansion, particularly in the temperature range from the growth temperature of the epitaxial layer to the device operating temperature, so that crystal stress can be minimized.
However, a problem arises since some known processes for forming epitaxial layers, as previously described, use a high temperature in order to provide increased molecular motion that facilitates achieving a crystalline orientation. As the process temperature is increased, the substrate and the deposited material expand at different rates and a thermal mismatch occurs, which hinders the formation of an epitaxial layer. In addition, it is often desirable to use a low process temperature on certain compound semiconductor materials which decompose at elevated temperatures, thus losing their surface crystalline quality and defeating epitaxial deposition thereon. A low temperature is also desirable in order to avoid diffusion of doped regions formed in the substrate, as well as diffusion of the substrate material into the epitaxial layer, as previously discussed.
The present invention is directed to the alleviation of these prior art problems of high processing temperature and undesirable morphology (e.g., smoothness, pinhole density, step coverage) of epitaxially deposited materials.